1. Field of the Invention
The invention relates to a method for diagnosing an exhaust gas treatment system for the reduction of nitrogen oxides in the exhaust gas train of an internal combustion engine by means of selective catalytic reduction, where a reducing agent is fed into the exhaust gas via a metering device, with at least one NOx-sensitive sensor located downstream of an exhaust gas treatment device, and further to a device for implementing the method. Furthermore, the invention relates to an internal combustion engine, in particular a diesel engine, with an exhaust train in which is located at least one exhaust gas treatment device, preferably a particulate filter, where regeneration of the exhaust gas treatment device may be initiated by increasing the exhaust gas temperature. The invention further relates to a method of operating an internal combustion engine.
2. The Prior Art
SCR-systems (Selective Catalytic Reduction) consist of a SCR-converter in the exhaust train of an internal combustion engine, which at sufficiently high temperatures catalyzes the reaction of nitrogen oxides (NOx) with a reducing agent, producing innocuous end products such as water and nitrogen. A system of this type comprises a container for the reducing agent, a metering device for the reducing agent, and an injection nozzle for the reducing agent, which injects the reducing agent into the exhaust gas before the SCR-converter. In practical applications an aqueous urea solution is commonly used as reducing agent, which at high temperatures in the SCR-converter releases ammonia (NH3) as the active substance.
The SCR-system is controlled by an electronic control unit ECU, which is either the control unit of the internal combustion engine itself or a separate control unit exchanging data with the control unit of the internal combustion engine. The control unit actuates the metering device for the reducing agent in such a way that the exhaust gas is supplied with the amount of reducing agent required for reducing the nitrogen oxides generated at the given operating point of the engine. In many cases a temperature sensor is provided in the exhaust train in order to ensure that the reducing agent is supplied only if the temperature of the SCR-converter is sufficiently high.
Since SCR-systems of this kind substantially reduce the amount of nitrogen oxides present in the exhaust train, it is necessary to monitor the correct functioning of the SCR-system as part of an “on-board diagnosis” program. In particular, it must be ascertained that the required amount of reducing agent has actually been supplied. Malfunctions which must be diagnosed are for instance clogging of the feeder nozzle or the filling of the reducing agent tank with liquids which do not have the required properties of a reducing agent, as for instance water without dissolved urea.
The objective of diagnosing an SCR-system could simply be achieved by providing a suitably sensitive NOx sensor downstream of the SCR-converter in the exhaust train, which transmits data on NOx concentration behind the SCR-converter to the control unit ECU. An arrangement of this kind is known from U.S. Pat. No. 6,363,771 B1. The sensitivity of commercially available NOx sensors is not sufficient, however, to perform the diagnosis task for all required engine operating points. If conventional NOx sensors are used, it may happen that in spite of a malfunction of the SCR-system leading to increased NOx concentration in the exhaust gas behind the SCR-converter, even this increased concentration will be too low for detection by the sensor so that the malfunction cannot be diagnosed.
From DE 102 61 877 A1 a method for operating an internal combustion engine with a particulate filter is known, wherein a particulate filter located in the exhaust train of the internal combustion engine is regenerated when necessary. During operation of the internal combustion engine a λ-value of a fuel-air mixture is changed periodically up and down over a predetermined period of time, between a value <1 and a value >1, such that the temperature of the particulate filter is increased by exhaust gas heating to a value at which regeneration of the particulate filter sets in.
JP 2003-161145 A discloses an exhaust gas treatment system with a NOx converter in the exhaust train and a throttle valve positioned downstream of the converter.
It is furthermore known to increase the temperature in the particulate filter by throttling the air mass flow in front of the intake manifold. This will reduce air throughput and excess air. With less excess air a given mass of fuel is burned with less air, thereby increasing the temperature of the combustion products. It is disadvantageous that throttling in front of the intake manifold will result in a considerable volume reduction, causing regeneration of the particulate filter to take longer.
It is the object of the present invention to describe a method which will permit diagnosis of exhaust gas treatment malfunctions even when NOx-sensors of low sensitivity are used. A further aim of the invention is to propose a device and a method for regeneration of an exhaust gas treatment device with short duration of the regeneration process.